select a.* from 
(
  select c_name,n_name from class,news where class.c_id = news.c_id
) a 
where n_name in (select top 20 n_name from 
  (
     select c_name,n_name from class,news where class.c_id = news.c_id
  ) b where c_name=a.c_name ) 
order by a.c_name,a.n_name

------解决方案--------------------
--每個類別下的20條？

select a.C_name,b.n_name from  class a,news b
where a.c_id=b.c_id
and b.n_id in (select top 20 n_id from news where c_id=a.c_id order by n_id desc)
order by a.c_name,b.n_id desc

------解决方案--------------------
SQL code
select n_id, n_name, C_name 
from (select * from news N where (select count(1) from news where c_id=N.c_id and n_id>N.n_id)<20) News
  join class on News.c_id=class.c_id
order by N.c_id,N.n_id desc